1. Field of the invention
This invention relates to a novel semiconductor laser device having window regions in which the absorption of laser light is suppressed and an epitaxial growth process for the production of the semiconductor by which the thickness of the grown layer can be controlled.
2. Description of the Prior Art
In order to achieve continuously stable laser oscillation in a single mode at a low threshold current level, semiconductor laser devices have been designed in a manner to define the active layer by cladding layers, and moreover to confine light and carriers with a high density within the laser oscillation area of the active layer by means of a current-confining striped structure and a waveguide structure. However, when the semiconductor laser devices operate at a high output power, the density of light in the active layer is increased, resulting in heat deterioration of the laser devices. To resolve this problem, TRS (a high-power, single-mode laser with a twin-ridge-substrate structure) lasers have been proposed (Appl. Phys. Lett., vol. 42, No. 10, 15th May, 1983, p. 853) in which a thin active layer is formed to decrease the light density of the active layer thereby attaining high output power operation. The thin active layer is grown on the ridges of the base layer by liquid phase epitaxy so as to control the thickness thereof. However, even though a thin active layer is grown on the ridges of the base layer and accordingly a decrease in the light density thereof is achieved, the absorption of laser light at or near the facets does not decrease, causing damage and/or deterioration of the facets, which shortens the life of these TRS lasers.
In general, as a crystal growth process by which a crystal layer with a nonuniform thickness in the direction from one end to the other end of the surface of a semiconductor substrate is grown on the substrate by liquid phase epitaxy, there is a process in which a mesa is formed on a semiconductor substrate and a crystal layer is grown on the substrate. However, when the height of the mesa is not sufficiently smaller than the thickness of the epitaxially grown layer, the thin portion of the epitaxially grown layer corresponding to the mesa cannot be formed into a flat-shape but it is formed into a convexed shape, which causes difficulty in the control of the thickness of the portion of the succeeding layer epitaxially grown on the said convexed portion so that the thickness of the said portion of the layer becomes thinner than that of the other portion of the layer. On the other hand, when the height of the mesa is sufficiently smaller than the thickness of the epitaxially grown layer, the whole surface of the grown layer becomes flat, so that the thickness of the succeeding layer becomes uniform over the whole area thereof regardless of the mesa of the substrate, which causes difficulty in the epitaxial growth of a crystal layer with a uniform thickness. Thus, semiconductor laser devices with a multi-layered structure having different layer thicknesses cannot be provided even though a conventional epitaxial growth process for the production of semiconductor laser devices is employed with liquid phase epitaxy.